This application is based on an application No. 2000-53785 filed in Japan, the content of which is hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to a semiconductor integrated circuit, a contactless information medium having the semiconductor integrated circuit, and a method for driving the semiconductor integrated circuit.
(2) Description of Related Art
Recently, contactless information mediums such as IC cards have been in practical use, where in the contactless information mediums, the mutual induction of the coil is used for data transfer and the power supply in the form of a radio wave having a certain wave length. The IC cards are classified broadly into the proximity type, the vicinity type and the like according to the distance between the IC card and a reader/writer with which they can communicate with each other. The standards are currently prepared for each type.
The proximity IC cards, which can be used at a distance from the reader/writers of approximately 1 cm to 20 cm, especially have a possibility of having a very broad range of uses. For example, people having an IC card as a season ticket can pass through the ticket collecting gate without taking out the card from the card case since the gate is opened or closed by communication between the IC card and the reader/writer in a contactless state.
However, for the IC cards to have a broad range of uses, it is important that the IC cards are compact and lightweight. In addition, it is expected that the wider range of uses the IC cards have, the more roughly the cards are handled. Therefore, taking into consideration the protection from such rough handling, it is a general rule that the contactless information mediums such as IC cards have a semiconductor integrated circuit containing a complicated circuit.
Now, the construction of a typical contactless IC card having a semiconductor integrated circuit will be described. FIG. 1 is a block diagram showing the construction of a typical contactless IC card. Note that FIG. 1 shows a reader/writer 990 which transmits/receives radio waves to/from an IC card 900, as well as the IC card 900. The following are description of the construction and operation of the IC card 900.
The IC card 900 includes an antenna coil 981 which is used to transmit/receive radio waves to/from an antenna coil 991 connected to the reader/writer 990. An alternating voltage is generated at both ends of the antenna coil 981 when the antenna coil 981 receives a radio wave from the antenna coil 991, and the generated alternating voltage is input into a semiconductor integrated circuit 910 contained in the IC card 900. 982 in the drawing indicates a capacitor for tuning.
The antenna coil 981 for reception is typically connected to the semiconductor integrated circuit 910 of the IC card 900. The capacitor 982 for tuning is often connected to the semiconductor integrated circuit 910 of the IC card 900, but in some cases it is placed in the semiconductor integrated circuit 910.
The IC card 900 receives ASK(Amplitude Shift Keying)-modulated signals from the reader/writer 990 and obtains power for driving the semiconductor integrated circuit 910 from the received signals, and also obtains data transferred from the reader/writer 990. FIG. 2 shows a specific example of the construction of the carrier wave transmitted from the reader/writer 990. As shown in the drawing, the parts having small amplitude in the ASK-modulated carrier represent data 0, and the parts having large amplitude data 1.
The semiconductor integrated circuit 910 includes a power supply circuit 911, a first regulator circuit 912, a modulation/demodulation circuit 913, a logic circuit 914, a nonvolatile memory 915, a step-down circuit 916, and a second regulator circuit 917. Note that the step-down circuit 916 may be replaced with a step-up circuit for a reason to be described later.
FIG. 3 shows the internal construction of the power supply circuit 911. As shown in the drawings, in the conventional IC card 900, a general all-wave rectifier circuit 9111 and a capacitor 9112 for smoothing constitute the power supply circuit 911. With this construction, an alternating voltage generated at both ends of the antenna coil 981 is rectified to become a direct voltage VCC. The rectified current is then regulated by the first regulator circuit 912 not to exceed a certain voltage value, and the regulated current is used as a voltage to drive the modulation/demodulation circuit 913 or the memory 915. The rectified current is also stepped down by the step-down circuit 916 and regulated by the second regulator circuit 917 not to exceed a certain voltage value, and the regulated current is used as a voltage to drive the logic circuit 914.
Though not shown in FIG. 1, the current having passed through the first regulator circuit 912 supplies a driving power to analog circuits such as a clock generator circuit. Here, the clock generator circuit generates a clock signal from the alternating voltage generated at both ends of the antenna coil 981, the clock signal being used for operating the logic circuit 914 and the nonvolatile memory 915.
Generally, digital circuits such as the logic circuit 914 are driven by a relatively low voltage (approximately 2V to 3V), while a voltage higher than this need to be supplied to the nonvolatile memory 915. For example, FeRAM requires approximately 3V to 7V of voltage, and EEPROM requires approximately 10V of voltage (for writing or erasing). To deal with this, in the conventional contactless IC card 900, the voltage generated by the power supply circuit 911 is stepped down by the step-down circuit 916 then supplied to the logic circuit 914. Alternatively, a low voltage for driving the logic circuit 914 may be generated by the power supply circuit 911, then the generated voltage may be stepped up by a step-up circuit to be used for driving analog circuits (such as the modulation/ demodulation circuit 913 and the clock generator circuit) and the nonvolatile memory 915.
As shown in FIG. 2, the data transferred between the IC card 900 and the reader/writer 990 is piggybacked onto the carrier wave. The data received by the contactless IC card 900 from the reader/writer 990 is demodulated by the modulation/ demodulation circuit 913; and the data to be transmitted from the contactless IC card 900 to the reader/writer 990 is modulated by the modulation/demodulation circuit 913. The data transferred between the contactless IC card 900 and the reader/writer 990 is controlled by the logic circuit 914 and stored in the nonvolatile memory 915.
Meanwhile, in the contactless IC card 900 in which the mutual induction of the coil is used to supply power and transmit/receive data, the power supply voltage generated by the power supply circuit 911 changes depending on the distance between the reader/writer 990 (power supply source) and the contactless IC card 900. A very short distance between them in particular may generate an overvoltage and destroy the internal circuits of the contactless IC card 900. To prevent such a failure, the first and second regulator circuits 912 and 917 are provided to regulate the power supply voltage generated by the power supply circuit 911 not to exceed a certain voltage value.
FIG. 4 shows the construction of a circuit conventionally used as the first regulator circuit 912. First and second P-channel MOS transistors (hereinafter referred to as PchMOS transistors) 931 and 932 are connected in series between the output from the power supply circuit 911 (represented as xe2x80x9cVCCxe2x80x9d in the drawing) and the ground. The gate and the drain of the first PchMOS transistor 931 are directly connected to each other, and the source of the first PchMOS transistor 931 is connected to VCC.
The drain of the first PchMOS transistor 931 is connected to the source of the second PchMOS transistor 932. The gate and the drain of the second PchMOS transistor 932 are connected to the output of a reference voltage generating circuit 933 and the ground, respectively. A node placed between the first and second PchMOS transistors 931 and 932 is connected to the base of a first PNP-type bipolar transistor 934. The collector of the first PNP-type bipolar transistor 934 is connected to the ground, and the emitter of the first PNP-type bipolar transistor 934 is connected to VCC via a resistor 935. The emitter of the first PNP-type bipolar transistor 934 is also connected to the base of a second PNP-type bipolar transistor 936, and the collector of the second PNP-type bipolar transistor 936 is connected to the ground. The emitter of the second PNP-type bipolar transistor 936 is output as a power supply (represented as VDD in the drawing) to the modulation/demodulation circuit 913 or the nonvolatile memory 915.
Now, the operation of the first regulator circuit 912 will be described. When it supposed that the threshold voltage at the second PchMOS transistors 932 of the first regulator circuit 912 is represented as VGS, that the voltage between the base and the emitter of the first PNP-type bipolar transistor 934 is represented as VBE1, that the voltage between the base and the emitter of the second PNP-type bipolar transistor 936 is represented as VBE2, and that the reference voltage generated by the reference voltage generating circuit 933 is represented as xe2x80x9cVrefxe2x80x9d, then when the voltage VCC output from the power supply circuit 911 exceeds a value (Vref+VGS+VBE1+VBE2), the PNP-type bipolar transistors are tuned ON to decrease the voltage VDD output from the regulator circuit to (Vref+VGS+VBE1+VBE2). Hereinafter, a provisional maximum value (Vref+VGS+VBE1+VBE2) of the output voltage VDD regulated by the first regulator circuit 912 is represented as xe2x80x9cVmaxxe2x80x9d. The details of the maximum voltage control are as follows.
The output of the reference voltage generating circuit 933 is input into the gate of the second PchMOS transistors 932. As a result, the gate voltage is Vref. When it is supposed that the threshold value of the second PchMOS transistors 932 is represented as VGS, the source voltage of the second PchMOS transistors 932 is (Vref+VGS). When the source voltage exceeds this value, the second PchMOS transistors 932 is tuned ON to decrease the source voltage to (Vref+VGS). On the other hand, when the source voltage of the second PchMOS transistors 932 is less than (Vref+VGS), the second PchMOS transistors 932 is tuned OFF and the current does not flow, and the source voltage is increased to (Vref+VGS) by the current sent from the drain of the first PchMOS transistors 931. As a result, in either cases, the source voltage of the second PchMOS transistors 932 becomes (Vref+VGS) eventually.
Now, the operation of the first PchMOS transistors 931 will be described. As described above, the drain of the first PchMOS transistor 931 is connected to the source of the second PchMOS transistor 932, and the gate and the drain of the first PchMOS transistor 931 are connected to each other. Since the source voltage of the second PchMOS transistor 932 is (Vref+VGS), the gate voltage of the first PchMOS transistor 931 is (Vref+VGS). When it is supposed that the threshold voltage of the first PchMOS transistor 931 is represented as VGS2, the first PchMOS+ transistor 931 is turned ON when the voltage VCC exceeds (Vref+VGS+VGS2).
Now, the operation of the first PNP-type bipolar transistor 934 will be described. As described earlier, the base voltage of the first PNP-type bipolar transistor 934 is (Vref+VGS). Since the emitter and the base of the first PNP-type bipolar transistor 934 is connected with the pn junction method, to pass the current through the area between the emitter and the base, the base-emitter voltage VBE1 is required to be higher than the forward voltage of the diode.
Accordingly, when the current is passing through the first PNP-type bipolar transistor 934, the emitter voltage is (Vref+VGS+VBE1).
The second PNP-type bipolar transistor 936 will be described. The emitter of the first PNP-type bipolar transistor 934 is connected to the base of the second PNP-type bipolar transistor 936. Accordingly, the base voltage of the second PNP-type bipolar transistor 936 is (Vref +VGS +VBE1). Here, when the base-emitter voltage of the second PNP-type bipolar transistor 936 is represented as VBE2, the emitter voltage of the second PNP-type bipolar transistor 936 is the Vmax (=Vref+VGS+VBE1+VBE2) considering in the same way as the first PNP-type bipolar transistor 934.
When the emitter voltage exceeds Vmax, the second PNP-type bipolar transistor 936 is turned ON and decreases the emitter voltage to Vmax. The emitter of the second PNP-type bipolar transistor 936 is the output from the first regulator circuit 912, and is also the source VDD supplied to the modulation/demodulation circuit 913 and the like. That is to say, the voltage VDD is regulated not to exceed Vmax.
When the voltage VCC supplied from the power supply circuit 911 is lower than Vmax, the second PNP-type bipolar transistor 936 is not turned ON. Therefore, the first regulator circuit 912 does not operate. The voltage VDD output from the first regulator circuit 912 becomes the same as the voltage VCC supplied from the power supply circuit 911.
As described earlier, when an ASK-modulated carrier is used for transferring data between the reader 990 and the contactless IC card 900, data 0 and 1 are defined in accordance with the level of the amplitude. The parts of the carrier having great amplitude are regarded as data 1 and the parts having small amplitude are regarded as data 0. As shown in FIG. 2, the parts of the carrier corresponding to data 0 actually have a certain level of amplitude, instead of having no amplitude. This arrangement is made for fear of failing to drive the contactless IC card 900 which is caused when the carrier is not sent due to succession of data 0 and the power supply voltage (VCC or VDD) is not generated.
Here, when the size of the small amplitude is close to the size corresponding to Vmax, the voltage VCC exceeds Vmax when the amplitude becomes large. This drives the first regulator circuit 912 and decreases the voltage VDD to Vmax. Accordingly, the difference between the voltage VDDs supplied to the modulation/demodulation circuit 913 when data is 0 and when data is 1 becomes smaller.
Whether the received data signal is 0 or 1 is judged when the modulation/demodulation circuit 913 demodulates the VDD. Therefore, when the difference between the voltage values corresponding to data 0 and data 1 becomes small, there is a possibility that the modulation/demodulation circuit 913 cannot judge the difference between data 0 and data 1, and a possibility that a malfunction might be caused when there is a noise in the signal.
Furthermore, when the size of the small amplitude is larger than the size corresponding to Vmax, the first regulator circuit 912 is always driven. When this happens, it is impossible to differentiate data 0 from data 1 from the VDD value after the signal has passed through the first regulator circuit 912.
In other words, when the distance between the reader/writer 990 and the contactless IC card 900 is too short, the voltage VDD corresponding to data 0 becomes high enough to make the discrimination between data 0 and data 1 difficult. When this happens, the data transmitted from the reader/writer 990 cannot be discriminated and cannot be written to the nonvolatile memory 915.
As described above, there is a problem that the data cannot be discriminated when the distance between the reader/writer 990 and the contactless IC card 900 is too short. However, in the contactless IC card which supplies electric power through an ASK-modulated radio waves, malfunctions that may be caused when the distance is too long need to be prevented.
In the above-described power supply circuit 911, the voltage generated by the all-wave rectifier circuit is step down by the step-down circuit, or step up by the step-up circuit so that a voltage to be supplied to the analog circuits or nonvolatile memory 915 and a voltage to be supplied to the logic circuit 914 are generated. However, there is another problem. With the above-described conventional method, the electricity supplied through radio waves cannot be fully used and the distance between the reader/writer 990 and the contactless IC card 900 with which the card can be used is short.
It is therefore the first object of the present invention to provide a semiconductor integrated circuit which supplies voltages with which discrimination between data 0 and data 1 is possible even in a circumstance in which the regulator regulates voltages so that the voltage of the data signal received by the modulation/demodulation circuit does not exceed a certain value.
It is the second object of the present invention to provide a semiconductor integrated circuit which effectively uses the power supplied from the reader/writer 990 and enables the contactless IC card 900 to communicate with the reader/writer 990 with a longer distance between them than conventional techniques.
The first object is fulfilled by a semiconductor integrated circuit comprising: a rectifier circuit which rectifies AC power to DC power; a regulator circuit which includes an input terminal for receiving the DC power, an output terminal, and a control terminal for receiving a reference voltage, and exercises control so that a voltage output from the output terminal does not exceed a voltage value determined from the reference voltage received by the control terminal; and a reference voltage changing circuit which changes the reference voltage received by the control terminal in correspondence to voltage change of the DC power.
With the above construction, the reference voltage input to the control terminal of the regulator circuit is changed in correspondence to the change in the voltage of the direct-current power rectified by the rectifier circuit. As a result, when the semiconductor integrated circuit is installed on a contactless information medium such as an IC card, it is possible to discriminate, from the output voltage, the changes of the data piggybacked onto the carrier even if the power supplied from the carrier has become overvoltage.
The second object is fulfilled by a contactless information medium comprising: a power generation circuit which receives an ASK-modulated carrier from outside the contactless information medium and generates AC power; a rectifier circuit which rectifies the AC power generated by the power generation circuit to DC power; a reference voltage generation circuit which outputs a reference voltage; a regulator circuit which includes an input terminal for receiving the DC power, a control terminal, and an output terminal, regulates the DC power so as not to exceed a voltage value determined from a voltage value received by the control terminal, and outputs the regulated DC power from the output terminal; and a reference voltage changing circuit which changes the reference voltage in correspondence to voltage change of the DC power, the changed reference being input to the control terminal.
With the above construction, the two-voltage rectifier circuit outputs two direct-current powers with different voltage values in parallel. This improves, for example, the use efficiency of the driving power supplied by the carrier. Therefore, the contactless information medium operates with more stability than conventional techniques even when the power supply source is distant from it, resulting in a longer distance between itself and a reader/writer, than conventional techniques, with which they can communicate with each other.